It is known that the response of the visual system is affected by the stimuli, which it receives. The threshold for such stimulation varies between individuals and, under adverse conditions, can significantly reduce performance. When the visual system is over stimulated, it reacts in a number of ways. Amongst a variety of undesirable effects, which can be caused, two examples include a drop in convergence sufficiency and a reduction in the ability to accommodate or fuse images. In addition, visual dyslexia may become apparent and migraines can be caused. Visual dyslexia is a condition of impaired reading and writing ability due to visual perception or visualisation problems. It is apparent therefore that for some it is necessary to modify the visual stimulus by changing the spectral distribution in a specific task e.g. reading and writing in school. In summary, it is well established that the colour of ambient lighting has a major influence on the effects of disorders such as dyslexia, epilepsy and migraine. In the case of dyslexia some sufferers can alleviate their reading problems by covering the page with a transparent coloured overlay in order to block out those wavelengths of light which give rise to an aspect of their problem. These overlays typically remove various amounts of simple primary colours, such as red, green or blue light and whilst they may assist with reading, they are of no value for writing.
In U.S. Pat. No. 5,855,428 (Wilkins) apparatus is described in which the spectral distribution of light from a fluorescent lamp to illuminate a surface to support reading material is altered by the interposition of specifically selected broadband filters. By adjustment of the position of the selected filter or filters different colours and saturation thereof can be selected.
In U.S. Patent Application No 2001/0005319 A1 (Ohishi et al.) an illumination control system, for general use, is described, in which the coordinates in colour space of the controlled illumination are arranged to follow a predetermined locus of points by mixing specific amounts of light from a plurality of differently coloured light emitting diodes (LED's).
Neither of these documents identifies the benefit of using sources which are characterised by providing light with a spectral distribution which is relatively narrow for application to the alleviation of the symptoms of the physiological defects and/or pathological conditions identified herein. This would be the case for laser sources, super-luminescent LED's and conventional coloured LED's, which provide light with a typical spectral bandwidth of between 17 nm to around 50 nm. The provision of illumination using additive light sources, such as LED's for the quantitative diagnosis and alleviation of the symptoms identified is the subject of this invention.